From Googling: “Tickle a dog’s tummy, and you could get a knee-jerk reaction. There is a certain area on a dog’s body that is referred to as the “saddle region.” It vaguely consists of the back, sides and flanks of the pooch. Rubbing or scratching a dog in the saddle region (the exact spot varies according to the individual pet) will cause the hind leg to simulate a scratching motion. This is called the “scratch reflex.” The dog’s spinal nerves pass all the way to his chest and abdomen, and stimulation of these nerves will cause a feeling of “itchiness” (similar to the irritation of a flea) somewhere on his body, causing the rear leg to involuntarily attempt to scratch it. The leg extends to different lengths or reaches to varying heights depending upon the area that is stimulated. Veterinarians who suspect spinal or neck damage in canines use the scratch reflex as a diagnostic tool, much like when doctors tap humans just below the kneecap to test nerve reactions.”